Method and apparatus for creating a bi-polar virtual electrode used for the ablation of tissue

ABSTRACT

A method and apparatus for creating a virtual electrode to ablate bodily tissue. The apparatus includes an outer tube, a first electrode, an inner tube and a second electrode. The outer tube is fluidly connected to a source of conductive fluid and defines a proximal end and a distal end. The distal end includes an opening for delivering conductive fluid from the outer tube. The first electrode is disposed at the distal end of the outer tube for applying a current to conductive fluid delivered from the outer tube. The inner tube is coaxially received within the outer tube and is connected to a source of conductive fluid. The inner tube defines a proximal end and a distal end, with the distal end forming an opening for delivering conductive fluid from the inner tube. Finally, the second electrode is disposed at the distal end of the inner tube for applying a current to conductive fluid delivered from the inner tube. With this configuration, upon final assembly, the distal end of the outer tube is axially spaced from the distal end of the inner tube such that the first electrode is spaced from the second electrode. As a result, a bi-polar virtual electrode can be established.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/347,971, entitled “Method and Apparatus forCreating a Bi-Polar Virtual Electrode Used for the Ablation of Tissue”filed on Jul. 6, 1999. In addition, this application claims the benefitof U.S. Provisional Application No. 60/091,929, filed on Jul. 7, 1998.

FIELD OF THE INVENTION

[0002] The present invention relates generally to an apparatus forcreating a virtual electrode. More particularly, the present inventionrelates to an apparatus for the creation of a virtual electrode that isuseful for the ablation of soft tissue and neoplasms.

BACKGROUND OF THE PRESENT INVENTION

[0003] The utilization of an electric current to produce an ameliorativeeffect on a bodily tissue has a long history, reportedly extending backto the ancient Greeks. The effects on bodily tissue from an appliedelectric current, and thus the dividing line between harmful andcurative effects, will vary depending upon the voltage levels, currentlevels, the length of time the current is applied, and the tissueinvolved. One such effect resulting from the passage of an electriccurrent through tissue is heat generation.

[0004] Body tissue, like all non-superconducting materials, conductscurrent with some degree of resistance. This resistance createslocalized heating of the tissue through which the current is beingconducted. The amount of heat generated will vary with the power Pdeposited in the tissue, which is a function of the product of thesquare of the current I and the resistance R of the tissue to thepassage of the current through it (P=I²R.).

[0005] As current is applied to tissue, then, heat is generated due tothe inherent resistance of the tissue. Deleterious effects in the cellsmaking up the tissue begin to occur at about 42° Celsius. As thetemperature of the tissue increases due to heat generated by thetissue□s resistance, the tissue will undergo profound changes andeventually, as the temperature becomes high enough, that is, generallygreater than 45° C., the cells will die. The zone of cell death is knownas a lesion and the procedure followed to create the lesion is commonlycalled an ablation. As the temperature increases beyond cell deathtemperature, complete disintegration of the cell walls and cells causedby boiling off of the tissue's water can occur. Cell death temperaturescan vary somewhat with the type of tissue to which the power is beingapplied, but generally will begin to occur within the range of 45° to60° C., though actual cell death of certain tissue cells may occur at ahigher temperature.

[0006] In recent times, electric current has found advantageous use insurgery, with the development of a variety of surgical instruments forcutting tissue or for coagulating blood. Still more recently, the use ofalternating electric current to ablate, that is, kill, various tissueshas been explored. Typically, current having a frequency from about 3kilohertz to about 300 gigahertz, which is generally known asradiofrequency or radiofrequency (RF) current, is used for thisprocedure. Destruction, that is, killing, of tissue using an RF currentis commonly known as radiofrequency ablation. Often radiofrequencyablation is performed as a minimally invasive procedure and is thusknown as radiofrequency catheter ablation because the procedure isperformed through and with the use of a catheter. By way of example,radiofrequency catheter ablation has been used to ablate cardiac tissueresponsible for irregular heartbeats or arrythmias.

[0007] The prior art applications of current to tissue have typicallyinvolved applying the current using a dry electrode. That is, a metalelectrode is applied to the tissue desired to be affected and agenerated electric current is passed through the electrode to thetissue. A commonly known example of an instrument having such anoperating characteristic is an electrosurgical instrument known as abovie knife. This instrument includes a cutting/coagulating bladeelectrically attached to a current generator. The blade is applied tothe tissue of a patient and the current passes through the blade intothe tissue and through the patients body to a metal base electrode orground plate usually placed underneath and in electrical contact withthe patient. The base electrode is in turn electrically connected to thecurrent generator so as to provide a complete circuit.

[0008] As the current from the bovie knife passes from the blade intothe tissue, the resistance provided by the tissue creates heat. In thecutting mode, a sufficient application of power through the bovie knifeto the tissue causes the fluid within the cell to turn to steam,creating a sufficient overpressure so as to burst the cell walls. Thecells then dry up, desiccate, and carbonize, resulting in localizedshrinking and an opening in the tissue. Alternatively, the bovie knifecan be applied to bleeding vessels to heat and coagulate the bloodflowing therefrom and thus stop the bleeding.

[0009] As previously noted, another use for electrical instruments inthe treatment of the body is in the ablation of tissue. To expandfurther on the brief description given earlier of the ablation ofcardiac tissue, it has long been known that a certain kind of hearttissue known as sino-atrial and atrio-ventricular nodes spontaneouslygenerate an electrical signal that is propagated throughout the heartalong conductive pathways to cause it to beat. Occasionally, certainheart tissue will misfire, causing the heart to beat irregularly. If theerrant electrical pathways can be determined, the tissue pathways can beablated and the irregular heartbeat remedied. In such a procedure, anelectrode is placed via a catheter into contact with the tissue and thencurrent is applied to the tissue via the electrode from a generator ofRF current. The applied current will cause the tissue in contact withthe electrode to heat. Power will continue to be applied until thetissue reaches a temperature where the heart tissue dies, therebydestroying the errant electrical pathway and the cause of the irregularheartbeat.

[0010] Another procedure using RF ablation is transurethral needleablation, or TUNA, which is used to create a lesion in the prostategland for the treatment of benign prostatic hypertrophy (BPH) or theenlargement of the prostate gland. In a TUNA procedure, a needle havingan exposed conductive tip is inserted into the prostate gland andcurrent is applied to the prostate gland via the needle. As notedpreviously, the tissue of the prostate gland heats locally surroundingthe needle tip as the current passes from the needle to the baseelectrode.

[0011] A lesion is created as the tissue heats and the destroyed cellsmay be reabsorbed by the body, infiltrated with scar tissue, or justbecome non-functional.

[0012] While there are advantages and uses for such dry electrodeinstruments, there are also several notable disadvantages. One of thesedisadvantages is that during a procedure, coagulum—dried blood cells andtissue cells—will form on the electrode engaging the tissue. Coagulumacts as an insulator and effectively functions to prevent currenttransfer from the blade to the tissue. This coagulum insulation can beovercome with more voltage so as to keep the current flowing, but onlyat the risk of arcing and injuring the patient. Thus, during surgerywhen the tissue is cut with an electrosurgical scalpel, a build-up ofcoagulated blood and desiccated tissue will occur on the blade,requiring the blade to be cleaned before further use. Typically,cleaning an electrode/scalpel used in this manner will involve simplyscraping the dried tissue from the electrode/scalpel by rubbing thescalpel across an abrasive pad to remove the coagulum. This is a tediousprocedure for the surgeon and the operating staff since it requires thereal work of the surgery to be discontinued while the cleaning operationoccurs. This procedure can be avoided with the use of specially coatedblades that resist the build up of coagulum. Such specialty blades arecostly, however.

[0013] A second disadvantage of the dry electrode approach is that theelectrical heating of the tissue creates smoke that is now known toinclude cancer-causing agents. Thus, preferred uses of such equipmentwill include appropriate ventilation systems, which can themselvesbecome quite elaborate and quite expensive.

[0014] A further, and perhaps the most significant, disadvantage of dryelectrode electrosurgical tools is revealed during cardiac ablationprocedures. During such a procedure, an electrode that is otherwiseinsulated but having an exposed, current carrying tip is inserted intothe heart chamber and brought into contact with the inner or endocardialside of the heart wall where the ablation is to occur. The current isinitiated and passes from the current generator to the needle tipelectrode and from there into the tissue so that a lesion is created.Typically, however, the lesion created by a single insertion isinsufficient to cure the irregular heartbeat because the lesion createdis of an insufficient size to destroy the errant electrical pathway.Thus, multiple needle insertions and multiple current applications arealmost always required to ablate the errant cardiac pathway, prolongingthe surgery and thus increasing the potential risk to the patient.

[0015] This foregoing problem is also present in TUNA procedures, whichsimilarly require multiple insertions of the needle electrode into theprostate gland. Failing to do so will result in the failure to create alesion of sufficient size otherwise required for beneficial results. Aswith radiofrequency catheter ablation of cardiac tissue, then, theability to create a lesion of the necessary size to alleviate BPHsymptoms is limited and thus requires multiple insertions of theelectrode into the prostate.

[0016] A typical lesion created with a dry electrode using RF currentand a single insertion will normally not exceed one centimeter indiameter. This small size—often too small to be of much or anytherapeutic benefit—stems from the fact that the tissue surrounding theneedle electrode tends to desiccate as the temperature of the tissueincreases, leading to the creation of a high resistance to the furtherpassage of current from the needle electrode into the tissue, all aspreviously noted with regard to the formation of coagulum on anelectrosurgical scalpel. This high resistance—more properly termedimpedance since typically an alternating current is being used—betweenthe needle electrode and the base electrode is commonly measured by theRF current generator. When the measured impedance reaches apre-determined level, the generator will discontinue current generation.Discontinuance of the ablation procedure under these circumstances isnecessary to avoid injury to the patient.

[0017] Thus, a typical procedure with a dry electrode may involveplacing the needle electrode at a first desired location; energizing theelectrode to ablate the tissue; continue applying current until thegenerator measures a high impedance and shuts down; moving the needle toa new location closely adjacent to the first location; and applyingcurrent again to the tissue through the needle electrode. This cycle ofelectrode placement, electrode energization, generator shut down,electrode re-emplacement, and electrode re-energization, will becontinued until a lesion of the desired size has been created. As noted,this increases the length of the procedure for the patient.Additionally, multiple insertions increases the risk of at least one ofthe placements being in the wrong location and, consequently, the riskthat healthy tissue may be undesirably affected while diseased tissuemay be left untreated. The traditional RF ablation procedure of using adry ablation therefore includes several patient risk factors that bothpatient and physician would prefer to reduce or eliminate.

[0018] The therapeutic advantages of RF current could be increased if alarger lesion could be created safely with a single positioning of thecurrent-supplying electrode. A single positioning would allow theprocedure to be carried out more expeditiously and more efficiently,reducing the time involved in the procedure. Larger lesions can becreated in at least two ways. First, simply continuing to apply currentto the patient with sufficiently increasing voltage to overcome theimpedance rises will create a larger lesion, though almost always withundesirable results to the patient. Second, a larger lesion can becreated if the current density, that is, the applied electrical energy,could be spread more efficiently throughout a larger volume of tissue.Spreading the current density over a larger tissue volume wouldcorrespondingly cause a larger volume of tissue to heat in the firstinstance. That is, by spreading the applied power throughout a largertissue volume, the tissue would heat more uniformly over a largervolume, which would help to reduce the likelihood of generator shutdowndue to high impedance conditions. The applied power, then, will causethe larger volume of tissue to be ablated safely, efficiently, andquickly.

[0019] Research conducted under the auspices of the assignee of thepresent invention has focused on spreading the current densitythroughout a larger tissue volume through the creation, maintenance, andcontrol of a virtual electrode within or adjacent to the tissue to beablated. A virtual electrode can be created by the introduction of aconductive fluid, such as isotonic or hypertonic saline, into or ontothe tissue to be ablated. The conductive fluid will facilitate thespread of the current density substantially equally throughout theextent of the flow of the conductive fluid, thus creating an electrode—avirtual electrode—substantially equal in extent to the size of thedelivered conductive fluid. RF current can then be passed through thevirtual electrode into the tissue.

[0020] A virtual electrode can be substantially larger in volume thanthe needle tip electrode typically used in RF interstitial ablationprocedures and thus can create a larger lesion than can a dry, needletip electrode. That is, the virtual electrode spreads or conducts the RFcurrent density outward from the RF current source—such as a currentcarrying needle, forceps or other current delivery device—into or onto alarger volume of tissue than is possible with instruments that rely onthe use of a dry electrode. Stated otherwise, the creation of thevirtual electrode enables the current to flow with reduced resistance orimpedance throughout a larger volume of tissue, thus spreading theresistive heating created by the current flow through a larger volume oftissue and thereby creating a larger lesion than could otherwise becreated with a dry electrode.

[0021] While the efficacy of RF current ablation techniques using avirtual electrode has been demonstrated in several studies, thecurrently available instruments useful in such procedures lags behindthe research into and development of hoped-for useful treatmentmodalities for the ablation of soft tissue and malignancies.

[0022] It would be desirable to have an apparatus capable of creating avirtual electrode for the controlled application of tissue ablating RFelectric current to a tissue of interest so as to produce a lesion ofdesired size and configuration.

SUMMARY OF THE INVENTION

[0023] One aspect of the present invention provides a surgical apparatusfor creating a virtual electrode to ablate bodily tissue. The surgicalapparatus comprises an outer tube, a first electrode, an inner tube anda second electrode. The outer tube is fluidly connected to a source ofconductive fluid and defines a proximal end and a distal end. In thisregard, the distal end of the outer tube includes an opening fordelivering a conductive fluid from the outer tube. The first electrodeis disposed at the distal end of the outer tube and is configured toapply a current to conductive fluid delivered from the outer tube. Theinner tube is coaxially received within the outer tube. The inner tubeis fluidly connected to a source of conductive fluid and defines aproximal end and a distal end. In this regard, the distal end of theinner tube forms an opening for delivering a conductive fluid from theinner tube. The second electrode is disposed at the distal end of theinner tube. The second electrode is configured to apply a current toconductive fluid delivered from the inner tube. Upon final assembly, thedistal end of the outer tube is axially spaced from the distal end ofthe inner tube such that the first electrode is spaced from the secondelectrode. With this configuration, then, a bi-polar virtual electrodecan be established.

[0024] Another aspect of the present invention provides a surgicalsystem for creating a virtual electrode to ablate bodily tissue. Thesurgical system includes a fluid source, a current source, and asurgical instrument. The fluid source maintains a supply of conductivefluid. The current source is configured to selectively supply anelectrical current. The surgical instrument includes an outer tube, afirst electrode, an inner tube and a second electrode. The outer tube isfluidly connected to the fluid source and defines a proximal end and adistal end. The distal end of the outer tube includes an opening fordelivering the conductive fluid. The first electrode is disposed at thedistal end of the outer tube and is electrically connected to thecurrent source. The inner tube is coaxially received within the outertube and is fluidly connected to the fluid source. The inner tubedefines a proximal end and a distal end, with the distal end of theinner tube forming an opening for delivering the conductive fluid. Thesecond electrode is disposed at the distal end of the inner tube and iselectrically connected to the current source. Upon final assembly, thedistal end of the outer tube is spaced from the distal end of the innertube such that the conductive fluid is delivered as a first bolus fromthe outer tube and a second bolus from the inner tube. Current isapplied to the first and second boluses by the first and secondelectrodes, respectively. With this configuration, a bi-polar virtualelectrode can be created.

[0025] Another aspect of the present invention relates to a method forablating bodily tissue at a target site. The method includes deliveringa first bolus of a conductive fluid at the target site. A second bolusof conductive fluid is also delivered at the target site, the secondbolus being spaced from the first bolus. Finally, a current issubstantially simultaneously applied to each of the first bolus and thesecond bolus to create a virtual electrode, ablating tissue in contactwith the first and second boluses. In one preferred embodiment, tissuebetween the first and second boluses is collapsed prior to applying thecurrent.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is a block diagram of a system for creating a virtualelectrode in accordance with the present invention;

[0027]FIG. 2 is a perspective view, with a portion cut away, of asurgical apparatus in accordance with the present invention; and

[0028]FIG. 3 is a schematic view of a portion of the surgical apparatusof FIG. 2 ablating bodily tissue.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029]FIG. 1 illustrates in block form a system 10 for RF ablationuseful with the present invention. The system 10 includes a currentsource of radiofrequency alternating electric current 12, a fluid sourceof RF ablating fluid 14, including but not limited to saline and otherconductive solutions, and a surgical instrument 16 for delivering RFcurrent and ablation fluid to a tissue site (not shown) for ablationpurposes. In one preferred embodiment, the surgical instrument 16 isconnected to the current source 12 and the fluid source 14. It will beunderstood that the current source 12 and the fluid source 14 may becombined into a single operational structure controlled by anappropriate microprocessor for a controlled delivery of ablating fluidand a controlled application of RF current, both based upon measuredparameters such as but not limited to, flow rate, tissue temperature atthe ablation site and at areas surrounding the ablation site, impedance,the rate of change of the impedance, the detection of arcing between thesurgical instrument and the tissue, the time period during which theablation procedure has been operating, and additional factors asdesired.

[0030] While the surgical instrument 16 is shown as being connected toboth the current source 12 and the fluid source 14, the present systemis not so limited but could include separate needles or otherinstruments useful in RF liquid ablation procedures, that is, forexample, a single straight or coiled needle having an exposed end and afluid flow path there through could be used to deliver both fluid andcurrent to the target tissue for ablation purposes. Alternatively, aseparate needle could be used to deliver the current and a separateneedle or needles could be used to deliver fluid to the target tissue.In addition, the application of the present system is not limited to theuse of straight needles or helical needles as surgical instruments butcould find use with any type of instrument wherein a conductive solutionis delivered to a tissue and an RF current is applied to the tissuethrough the conductive fluid. Such instruments thus would includestraight needles, helical needles, forceps, roller balls, instrumentsfor the treatment of vascular disorders, and any other instrument.

[0031] In one preferred embodiment, the system 10 further includes asecond fluid source 18 for delivery of tissue protecting fluid, via adelivery instrument 20, to a tissue whose ablation is not desired.

[0032] The surgical instrument 16 may assume a wide variety of forms.One preferred embodiment of a surgical apparatus 50 is shown in FIG. 2.The apparatus 50 generally includes a bi-polar electrode that is usefulin an ablation procedure using an RF ablating fluid such as, but notlimited to, isotonic or hypertonic saline. Further, the apparatus 50generally includes a plurality of coaxial thin walled tubes forming acatheter delivery system for RF ablating fluid and RF ablating current.Thus, the apparatus 50 preferably includes a first or outer thin walledtube 52 having a proximally attached hemostasis valve 54. The valve 54includes an inlet 56 for RF ablating fluid flow as indicated by arrow 58and also an access port 60 for an electrical line 62, which can beelectrically connected to the RF current source 12 (FIG. 1). A distalend of the first tube 52 may be provided with one or more thermocouplesor other temperature sensors 64. The distal end of the first tube 52also includes an electrode 66 which is electrically connected to theline 62, thereby providing one of the two bi-polar electrodes envisionedby a preferred embodiment of the present invention. It will beunderstood that the first tube 52 is electrically insulated or otherwisenon-conductive. The first tube 52 provides a flow passage for the RFablating fluid from the fluid source 14 (FIG. 1) to the distal end ofthe first tube 52 where it exits the first tube 52 as indicated byarrows 68.

[0033] The apparatus 50 further includes a second or intermediate thinwalled tube 70 coaxially disposed within the first tube 52. The secondtube 70 has a proximally attached hemostasis valve 72. The valve 72includes a suction port 74 for directing fluid flow as indicated byarrow 76. A distal end of the second tube 70 may be provided with one ormore vacuum apertures 78 through which a suction is applied tosurrounding tissue (not shown), via a vacuum formed at the suction port74. Thus, applying suction to the suction port 74 will draw thesurrounding tissue into contact with the vacuum apertures 78 and thedistal end of the second tube 70. In addition, with this same process,some RF ablating fluid may be removed via the applied suction asindicated by arrows 80.

[0034] The apparatus 50 further includes a third or inner thin walledtube 82 coaxially disposed within the second tube 70. The third tube 82has a proximally attached hemostasis valve 84. The valve 84 includes aninlet 84 for RF ablating fluid flow as indicated by arrow 86 and also anaccess port 88 for an electrical line 90, which can be electricallyconnected to the current source 12 (FIG. 1). A distal end of the thirdtube 82 also includes an electrode 92, which is electrically connectedto the line 90, thereby providing the other one of the two bi-polarelectrodes envisioned by a preferred embodiment of the presentinvention. It will be understood that the third tube 82 is electricallyinsulated or otherwise non-conductive. THE third tube 82 provides a flowpassage for the RF ablating fluid from the fluid source 14 (FIG. 1) tothe distal end of the third tube 82 where it exits the third tube 82 asindicated by arrows 94.

[0035] The surgical apparatus 50 may further include a probe 100 thatextends through an interior passage of the third tube 82 and that isfreely movable therewithin. The probe 100 may include a thermocouple 102disposed at a most distal end thereof and may be connected via anelectrical line 104 to the RF current source 12 (FIG. 1) to provide atemperature measurement at a predetermined distance from the electrodes66 and 92.

[0036] In operation, following delivery to a particular target site, RFablating fluid will be provided to the ports 56 and 84 from the fluidsource 14 (FIG. 1). The fluid will exit the distal ends of the firsttube 52 and the third tube 82, respectively, and begin to form bolusesof fluid along or within the tissue at the target site. Application of asuction at the vacuum aperture 78, via the suction port 74, will causethe tissue surrounding the distal end of the second tube 70 to becollapsed or pulled into a substantially fluid tight relationship withthe second tube 70, thereby preventing migration of the fluid, inparticular via a capillary effect, along the second tube 70 between thedistal ends of the first and third tubes 52, 82. RF ablating power canbe applied and an ablation procedure can be carried out. By theapplication of suction to the tissue and the prevention of the flow offluid along the second tube 70, shorting of the current between theelectrodes 66 and 92 can be avoided.

[0037]FIG. 3 represents schematically the fluid flow and current flowachieved during use of the present invention. Thus, FIG. 3 shows atissue 110, such as liver, into which a portion of the surgicalapparatus 50 has been inserted. Infusion of the RF ablating fluid willinitially create two separate boluses of fluid, 112 and 114. If not forthe suction applied to the tissue surrounding vacuum apertures 78, fluidwould tend to travel along the path created during insertion andplacement of the apparatus 50 in the tissue 110. Applying suctionintermediate the release of the fluid, however, prevents such fluidtravel and substantially prevents any shortage between the electrodes66, 92 directly therebetween. The applied current can thus spreadreadily through the boluses 112, 114 and then travel therebetween asindicated by the lines 116. In this way, then, ablation can beaccomplished between the bi-polar electrodes 66 and 92. The lesioncreated with the use of such a bi-polar structure will essentially bethe size of the boluses 112, 114 at a minimum, though it will beunderstood that cell-to-cell thermal conduction will occur that willmake the lesion in reality larger than the boluses 112, 114, assumingthat power is left on for the appropriate length of time.

[0038] As is well known, when an electro-surgical tool is used, or forthat matter, any electrical device, a complete circuit for current flowmust be provided. Thus, when a monopolar surgical instrument such as abovie knife is used, a ground pad connected to the RF current generatoris placed under the patient to provide a complete electrical circuitfrom the generator, to the knife, through the patient to the ground pad,and back to the generator. With the present invention, no ground pad isneeded. The current flows from the current source 12 (FIG. 1) to one ofthe two electrodes 66 or 92 on the apparatus 50, through the patient tothe other electrode 66 or 92, and then back to the current source 12.Such bi-polar structure enables the physician to localize the effect ofthe current passing through the patient, thereby avoiding certain risksassociated with the use of a monopolar device, including but not limitedto the risk of bums where the ground pad contacts the patient.Additionally, by controlling the relative spacing between the twoelectrodes 66, 92 of a bi-polar instrument, the size of the lesion canbe controlled. Thus, where the electrodes 66, 92 are close to eachother, there will be little tissue between them through which thecurrent will travel and the lesion size will be reduced relative to aninstrument where the electrodes are spaced relatively farther apart. Inaddition, the shape of the lesion is somewhat controllable bycontrolling both the fluid flow and the spacing of the electrodes 66,92. It will be understood that regardless of the device used that thelesion size and shape is a product of many factors, including the tissuecomposition, the conductivity of the fluid, the amount of appliedcurrent, the amount of cell-to-cell thermal conductivity, the timeperiod the current is applied, tissue temperature, and fluid temperaturewhen applied, among others.

[0039] Although the present invention has been described with referenceto preferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A surgical apparatus for creating a virtualelectrode to ablate bodily tissue, the apparatus comprising: an outertube fluidly connected to a source of conductive fluid, the outer tubedefining a proximal end and a distal end, the distal end of the outertube including an opening for delivering a conductive fluid from theouter tube; a first electrode disposed at the distal end of the outertube for applying a current to conductive fluid delivered from the outertube; an inner tube coaxially received within the outer tube, the innertube being fluidly connected to a source of conductive fluid anddefining a proximal end and a distal end, the distal end of the innertube forming an opening for delivering a conductive fluid from the innertube; and a second electrode disposed at the distal end of the innertube for applying a current to conductive fluid delivered from the innertube; wherein upon final assembly, the distal end of the outer tube isaxially spaced from the distal end of the inner tube such that the firstelectrode is spaced from the second electrode.
 2. The surgical apparatusof claim 1 , wherein the distal end of the outer tube is proximal thedistal end of the inner tube.
 3. The surgical apparatus of claim 1 ,further comprising: an intermediate tube coaxially disposed between theouter tube and the inner tube, the intermediate tube defining a proximalend and a distal end, wherein upon final assembly, the distal end of theintermediate tube is positioned between the distal end of the outer tubeand the distal end of the inner tube.
 4. The surgical apparatus of claim3 , wherein the intermediate tube is fluidly connected to a vacuumsource and forms a vacuum aperture at the distal end of the intermediatetube.
 5. The surgical apparatus of claim 4 , wherein the distal end ofthe intermediate tube forms a plurality of vacuum apertures each fluidlyconnected to the vacuum source.
 6. The surgical apparatus of claim 3 ,wherein the intermediate tube has a diameter less than diameter of theouter tube to permit flow of conductive fluid to the distal end of theouter tube.
 7. The surgical apparatus of claim 3 , wherein the distalend of the intermediate tube is configured to receive tissue during asuction procedure.
 8. The surgical apparatus of claim 3 , furthercomprising: an elongated probe slidably extending within the inner tube,the probe including a distal end such that upon final assembly, thedistal end of the probe selectively extends distal the distal end of theinner tube.
 9. A surgical system for creating a virtual electrode toablate bodily tissue, the surgical system comprising: a fluid sourcemaintaining a supply of a conductive fluid; a current source configuredto selectively supply an electrical current; and a surgical instrumentincluding: an outer tube fluidly connected to the fluid source, theouter tube defining a proximal end and a distal end, the distal end ofthe outer tube including an opening for delivering the conductive fluid,a first electrode disposed at the distal end of the outer tube andelectrically connected to the current source, an inner tube coaxiallyreceived within the outer tube, the inner tube being fluidly connectedto the fluid source and defining a proximal end and a distal end, thedistal end of the inner tube forming an opening for delivering theconductive fluid, a second electrode disposed at the distal end of theinner tube and electrically connected to the current source; whereinupon final assembly, the distal end of the outer tube is spaced from thedistal end of the inner tube such that the conductive fluid is deliveredfrom the outer tube and inner tube as a first and second bolus,respectively, to which a current is applied by the first and secondelectrodes, respectively.
 10. The surgical system of claim 9 , whereinthe distal end of the outer tube is proximal the distal end of the innertube.
 11. The surgical system of claim 9 , further comprising: anintermediate tube coaxially disposed between the outer tube and theinner tube, the intermediate tube defining a proximal end and a distalend, wherein upon final assembly, the distal end of the intermediatetube is positioned between the distal end of the outer tube and thedistal end of the inner tube.
 12. The surgical system of claim 11 ,wherein the intermediate tube is fluidly connected to a vacuum sourceand forms a vacuum aperture at the distal end thereof.
 13. The surgicalsystem of claim 12 , wherein the distal end of the intermediate tubeforms a plurality of vacuum apertures each fluidly connected to thevacuum source.
 14. The surgical system of claim 11 , wherein theintermediate tube has a diameter less than a diameter of the outer tubeto permit flow of conductive fluid to the distal end of the outer tube.15. The surgical system of claim 11 , wherein the surgical instrumentfurther includes: an elongated probe slidably extending within the innertube, the probe including a distal end such that upon final assembly,the distal end of the probe selectively extends distal the distal end ofthe inner tube.
 16. The method of ablating bodily tissue at a targetsite, the method comprising: delivering a first bolus of a conductivefluid at the target site; delivering a second bolus of a conductivefluid at the target site, the second bolus being spaced from the firstbolus; and substantially simultaneously applying a current to each ofthe first bolus and the second bolus to create a virtual electrode,ablating tissue in contact with the first and second boluses.
 17. Themethod of claim 16 , wherein the first and second boluses are deliveredsubstantially simultaneously.
 18. The method of claim 16 , furthercomprising: collapsing tissue between the first and second boluses priorto applying the current.
 19. The method of claim 18 , wherein collapsingthe tissue between the first and second boluses includes: positioning adistal end of a tube between the first and second boluses, the tubebeing connected to a vacuum source and the distal end forming a vacuumaperture; and activating the vacuum source to draw tissue into contactwith the distal end via the vacuum apertures.
 20. The method of claim 18, further including: suctioning excess conductive fluid from between thefirst and second boluses.